As US authorities scramble to find systems that can detect and prevent terrorists entering the country, biometric technology is at the heart of plans to secure its borders. Under the US-Visit scheme, most travellers entering the country on a visa now undergo photography and digital scanning of both index fingers.

The process is swift and painless. But with more than 300 points of entry to secure and plans to require foreign countries to issue biometric passports, the challenges remain daunting.

Contracts being awarded show the scale of US ambitions for border security. Last month Accenture, the management consultancy, secured a contract potentially worth $10 billion (€8 billion) - the largest awarded so far by the Department of Homeland Security - to build and manage a comprehensive border-control system.

Accenture and its sub-contractors will be considering all options, but Jim Williams, project manager for US-Visit, says the basic procedure will remain the capture of visitors' fingerprints and photographs. Secondary screenings - running data against vast government databases of known criminals and terrorists - are conducted if initial scans highlight an irregularity.

"Even if we use biometric passports in the future, we'll continue to use the digital finger scans. It's easy and a good check," says Mr Williams. "We're getting about one-tenth of 1 per cent of false positives, which we send to a fingerprint examiner who's on call 24 hours a day and can clear the case up in about three minutes."

Biometric technology uses an image to create a binary template that captures the distinct characteristics of a person's fingerprint, face or iris. For immigration authorities, its appeal lies in this ability to verify parts of the body, rather than simply confirming the presence of a valid document without verifying that the individual presenting it is its rightful owner.

The question remains of where to set an acceptance threshold. At 95 per cent, too many legitimate users are rejected; but set it too low and illegitimate users get through. "Getting a high number [of false acceptances and false rejections] is easy," says Joseph Kim, a senior consultant at the International Biometric Group, a New York-based research company. "Finding that fine line between is really the challenge."

The conditions in which images are acquired must also be controlled. "Fingerprinting doesn't work well with chapped and dry fingers," says Jim Wayman, director of biometric identification research at San Jose State University. "And facial recognition doesn't work well in varied lighting or background clutter. So in any of these systems, performance depends on how carefully we can control the acquisition environment."

Problems have also limited the adoption of iris recognition technology - the third biometric recommended by the International Civil Aviation Organisation. Contact lenses, wateriness and pale-blue eyes make the structure difficult to find; Asian eyelashes, which often curl inwards, can also interfere.

"If you have a population with huge open brown eyes that doesn't wear contact lenses and never cries, it's easy to capture it. But for a diverse population you have a challenge," says Joseph Atick, president and chief executive of Identix, a leading supplier of biometric technology.

Nevertheless, airports such as Frankfurt/Main have piloted the technology and, because of the iris's distinctive physiological features, some believe it holds promise, particularly for extensive searches. "To do a definitive one-to-many \ search, you have to take all 10 prints," says Mr Kim. "You need just one iris to search a very large database."

Vast databases of irises do not yet exist, which is largely why fingerprinting and facial recognition have so far won out. Capturing biometric data is useless without records against which they can be checked, and US government databases contain about 1.2 billion photographic images and 150 million fingerprints, according to Mr Atick, while iris images amount to only several hundred thousand.

Biometric technology is only half the battle - much of the task is operational. Take one gaping hole in immigration security: recording visitors' timely departure. Pilot schemes at Baltimore-Washington airport and Miami seaport operate kiosks where travellers must leave digital finger scans before departure. However, without physical exit channels that force people through scanning, compliance is proving tough.

With so many systems, the challenge is implementation on a vast scale while ensuring inter-operability across various systems and databases. "There has certainly never been any biometrics or IT system of this size and magnitude put together before," says Mr Wayman. "You could argue that ATM and credit-card systems are this big, but they carry less information."

Moreover, the US is driving action by other countries. US-Visit will require visitors from the 27 countries in the visa waiver programme to issue citizens with machine-readable passports containing biometric data. Scheduled to start in October, the scheme is likely to be delayed since most European countries say they are unable to meet that deadline.

"We have a long way to go even to conceptually have agreement from some countries about the information they put into this encoded piece of their passport," says John Szczygiel, head of Siemens Security Systems for North America. "And then the question is whether or not the standards are complementary, and I don't see a lot of progress in that area."

With so many players at home and abroad, Mr Williams admits the task ahead is as much organisational as technical. "We're looking at this as a business transformation," he says. "So it's about getting all the stakeholders within the public sector and private sector agreeing what is the right path to go down." - (Financial Times Service)